I'll make a suggestion, but it's up to you to try it.
First, download the tiny free app named "EasyFind":
A little collection of free utilities for the Mac to make your life easier.
www.devontechnologies.com
Open EasyFind and set it us thusly:
Search for -- files and folders
Operator -- Phrase
Comparison -- Ignore Case
Include -- Invisible files & folders
Next, check the popup menu that indicates WHERE you will be searching.
Set it up so it searches the entire "Macintosh HD" (unless you've given the drive another name)
Then, in the search bar, enter "garageband".
Click the magnifying glass to start the search and sit back for a while.
OK, did EasyFInd discover a bunch of left-over stuff from GB?
I'd go through the list and start deleting items.
BE AWARE that not all of them may "delete easily".
If you have an item that you can't just get rid of by clicking the delete button, try this:
Click ONE time on the item to select it
Type "command-R" to "reveal in the finder"
A finder window should open and the file should be highlighted.
Type "command-delete" to attempt to move it to the trash.
You will probably be asked for your administrative password, enter it.
Does the file disappear? GOOD, move on to the next one.
As you can see, this can be a job.
But step by step, it can be done.
Aside:
It probably would have been easier if you had used the free app "AppCleaner" to remove GB, but... you did what you did...